As is widely known, R—Fe—B based permanent magnets have high magnetic characteristics and thus are used in various fields today. Against such a background, in R—Fe—B based permanent magnet production plants, large amounts of magnets are produced every day. However, with an increase in the amount of magnets produced, the amounts of magnet scrap discharged as a defectively processed product or the like, magnet processing waste discharged as cutting waste, grinding waste, or the like, etc., during the production process have also been increasing. In particular, with the weight and size reduction of information devices, the size of magnets used therein has also been reduced, leading to an increase in the proportion of processing allowance, and, as a result, the production yield tends to decrease year by year. Accordingly, rather than discarding magnet scrap, magnet processing waste, and the like discharged during the production process, how to recover and recycle metallic elements contained therein, particularly rare earth elements, is an important technical challenge for the future. The same also applies to how to recover rare earth elements from electrical appliances in which R—Fe—B based permanent magnets are used, etc., and recycle them as recyclable resources.
Several methods have been proposed as methods for recovering a rare earth element from a workpiece containing at least a rare earth element and an iron group element. For example, Patent Document 1 proposes a method in which a workpiece is heated in an oxidizing atmosphere to convert the contained metallic elements into oxides, followed by mixing with water to form a slurry; hydrochloric acid is added with heating to dissolve a rare earth element in a solution; an alkali (sodium hydroxide, ammonia, potassium hydroxide, etc.) is added to the obtained solution with heating, thereby precipitating an iron group element leached into the solution together with the rare earth element; then the solution is separated from undissolved substances and the precipitate; and oxalic acid, for example, is added to the solution as a precipitant to recover the rare earth element in the form of an oxalate. This method is noteworthy as a method that allows a rare earth element to be effectively separated from an iron group element and recovered. However, the method has a problem in that because an acid and an alkali are used in part of the process, it is not easy to control the process, and also the recovery cost is high. Therefore, it must be said that in some aspects, the method described in Patent Document 1 is difficult to put into practical use as a recycling system that is required to be low-cost and simple.
In addition, as a method for not oxidizing an iron group element contained in a workpiece but oxidizing only a rare earth element contained in the workpiece to thereby separate the two, Patent Document 2 proposes a method in which a workpiece is heated in a carbon crucible. Unlike the method described in Patent Document 1, this method does not require an acid or an alkali. In addition, when a workpiece is heated in a carbon crucible, theoretically, the atmosphere in the crucible is autonomously controlled to an oxygen partial pressure at which iron group elements are not oxidized and only rare earth elements are oxidized. Accordingly, this method is likely to be more advantageous than the method described in Patent Document 1 in that the process is simpler. However, when it comes to the question whether the atmosphere in a carbon crucible is autonomously controlled to a predetermined oxygen partial pressure by merely heating a workpiece in the crucible, whereby rare earth elements can be separated from iron group elements, the reality is not necessarily so. Patent Document 2 states that the oxygen content of the atmosphere in a crucible is preferably 1 ppm to 1%, but essentially no external operation is required to control the atmosphere. However, according to the study by the present inventors, at least in the case where the oxygen content is less than 1 ppm, rare earth elements cannot be separated from iron group elements. Therefore, even if it is theoretically possible that when a workpiece is heated in a carbon crucible, the atmosphere in the crucible is autonomously controlled to an oxygen partial pressure at which iron group elements are not oxidized and only rare earth elements are oxidized, in reality, the inside of the crucible has to be artificially controlled to an atmosphere having an oxygen content of 1 ppm or more. Such control can be achieved by introducing an inert gas having an oxygen content of 1 ppm or more into a crucible, as also described in Patent Document 2. However, in the case of argon gas, which is widely used as an industrial inert gas, its oxygen content is usually 0.5 ppm or less. Therefore, for introducing argon gas having an oxygen content of 1 ppm or more into a crucible, the widely used argon gas cannot be directly used, and it is necessary to especially increase the oxygen content before use. Consequently, although the process of the method described in Patent Document 2 looks simple, actually it is not. It must be said that like the method described in Patent Document 1, in some aspects, the method described in Patent Document 2 is difficult to put into practical use as a recycling system that is required to be low-cost and simple.
Thus, as a method for recovering a rare earth element from a workpiece containing at least a rare earth element and an iron group element, which can be put into practical use as a low-cost, simple recycling system, the present inventors have proposed, in Patent Document 3, a method in which a workpiece is subjected to an oxidation treatment, then the treatment environment is turned into an environment where carbon is present, and an oxidation-treated workpiece is subjected to a heat treatment, whereby a rare earth element is separated in the form of an oxide from an iron group element and recovered.